Let's roll up our sleeves and solve this. We have 15 and we're going to multiply it by 12, essentially adding 15 to itself 12 times.
Step 1: 0 + 15 = 15
Step 2: 15 + 15 = 30
Step 3: 30 + 15 = 45
Step 4: 45 + 15 = 60
Step 5: 60 + 15 = 75
Step 6: 75 + 15 = 90
Step 7: 90 + 15 = 105
Step 8: 105 + 15 = 120
Step 9: 120 + 15 = 135
Step 10: 135 + 15 = 150
Step 11: 150 + 15 = 165
Step 12: 165 + 15 = 180

So, 15*12 = 180